Granulomatous colitis (GC) is a rare, breed-specific type of inflammatory bowel disease (IBD) condition occurring in Boxer dogs and occasionally French bulldogs.1,2 Affected dogs typically present with signs of colitis, hematochezia and weight loss, progressing to cachexia in severe cases. Until 2004, the mainstay of treatment for GC involved immunosuppression with agents such as corticosteroids and azathioprine, in combination with antibiotic therapy and dietary change. Responses to treatment were generally poor, frequently resulting in euthanasia. GC became considered an incurable, immune-mediated disease.3
Research at Cornell University using advanced molecular methods has demonstrated invasive E. coli packed within colonic macrophages of Boxer dogs with GC.1 In situ visualization of E. coli was achieved using FISH, fluorescent in situ hybridization, an imaging technique using fluorescent dyes attached to oligonucleotide probes that hybridize to bacterial 16S ribosomal DNA. Fluorescent labeling enables clear visualization of bacterial morphology and spatial localization against a busy background of severe inflammation.3,4 In > 90% of cases of GC investigated, E. coli is the sole bacteria invading colonic mucosa and residing intracellularly within colonic macrophages.5 Because GCB is breed specific and rare, an autosomal recessive genetic defect involving the immune system that confers susceptibility to E. coli invasion is suspected.3 The invading E. coli strains in GC are similar to a novel pathotype of E. coli in Crohn's disease, which have been shown to invade the intestinal mucosa of people, and survive and persist in mucosal macrophages. This pathotype has been named 'adherent invasive E. coli' (AIEC).7 Invasive GCB-associated E. coli persist in cultured epithelial cells for 48 h and appear to reside in a tight vacuole within the cytoplasm, suggesting a location in the endosomal lysosomal continuum.3
Diagnosis of GC is achieved by ruling out other causes of clinical signs, using faecal parasitology and culture, CBC, chemistry panel, exclusion of Addison's disease, and abdominal imaging, before proceeding to endoscopic biopsy. Routine clinicopathologic testing is usually unremarkable, but may reveal mild to moderate anemia due to chronic disease, or hemorrhage if hematochezia is very severe. The degree and chronicity of blood loss in GC can cause iron deficiency anemia, with microcytic and hypochromic erythrocytes. Hypoalbuminemia can also occur due to hemorrhage, protein exudation, anorexia, and inflammation (negative acute phase protein). Imaging studies (radiographs, ultrasound) are largely unremarkable, but useful to rule out other causes of tenesmus and hematochezia (e.g., partial intestinal obstruction, abdominal masses, chronic intussusception, lymph node enlargement, local organomegaly). Definitive diagnosis is achieved by histologic confirmation on colonic mucosal biopsies. Grossly, colonic mucosa is reddened, cobblestone, and ulcerated. Histology typical of GC reveals severe mucosal ulceration and infiltration of the submucosa and lamina propria with the pathognomonic finding of PAS+ macrophages.3
Demonstration of invasive E. coli in GC is integral to disease diagnosis and is achieved using FISH, performed on formalin-fixed, paraffin-embedded colonic mucosal biopsies (www.vet.cornell.edu/labs/simpson). The importance of appropriate antimicrobial selection in the treatment of GC cannot be overemphasized and has been demonstrated in a prospective study of 14 GCB.3 Empirical treatment without performing endoscopy and colon biopsy culture is not recommended, as the success of treatment is governed by the invading E. coli strain and its sensitivity to antimicrobials that concentrate intracellularly. We recommend culture of 2–3 colon biopsies, requesting that the microbiology lab performs E. coli enrichment and antimicrobial sensitivities for all cultured isolates. Enrofloxacin is usually the treatment of choice, but resistant E. coli have been isolated and are associated with poor response and often euthanasia.5 Currently, the suggested treatment regimen for cases with enrofloxacin sensitive E. coli is 5 mg/kg every 12 h for a minimum of 8 weeks.1 Cartilage defects are rarely appreciated, but concurrent treatment with glucosamine and chondroitin sulfate is advisable. A poor response to treatment is usually associated with enrofloxacin resistance, wherein repeat colonoscopic biopsy, FISH and culture are necessary to guide further treatment. In enrofloxacin-resistant cases, the antimicrobial selection should be determined by susceptibility testing.4 Aside from spectrum of activity, the antimicrobial used must be capable of penetrating macrophages; e.g., marbofloxacin, chloramphenicol, florfenicol, trimethoprim-sulfa, tetracyclines, clarithromycin, imipenem, meropenem.5
From a comparative aspect, granulomatous colitis also occurs in people, predominantly Crohn's disease (CD) and chronic granulomatous disease (CGD).1 CD is the main form of IBD occurring in people, and is a heterogenous group of disorders, resulting from the convergence of multiple factors: genetically determined susceptibility, altered immune tolerance of the enteric bacteria, and environmental triggers.8 CGD is a hereditary disease complex in which phagocytes have difficulty forming the superoxide radical used to kill certain ingested pathogens.9 Curiously, the colitis of CGD patients is almost identical to Boxer dog GC, including the unusual and pathognomonic finding of PAS+ foamy, lipid laden macrophages. Also in common with GC is that AIEC have been detected in the intestinal mucosa of people with CGD colitis (and ileal CD). AIEC were recovered from 100% of the biopsy specimens of early ileal CD lesions and 65% of chronically inflamed ileal resection.10 However, the precise role of AIEC in CGD and CD; i.e., whether a secondary invader or primary pathogen remains the subject of much debate, since they are present as resident flora in healthy individuals. Unlike Boxer dog GC, remission of disease after eradication of E. coli has not been demonstrated in CD or CGD.
AIEC are unique in that they do not possess any of the known virulence genes for invasion utilized by enteroinvasive or enteropathogenic E. coli, or Shigella strains.3 A functional change that brings about the proliferation and upregulation of virulence genes in AIEC has been suggested to account for their ability to invade and persist in colonic mucosal and macrophages of CD patients There is emerging evidence that AIEC utilize specific mechanisms to facilitate cellular invasion and survival, for example, flagellin, type I pili, cell adhesion molecule CEACAM6, which acts as a receptor for AIEC, the stress response protein Gp96, and long polar fimbriae.1,3,10 However, the means by which they evade the immune response in order to survive and replicate in macrophages remains the focus of much debate.
In conclusion, GCB is associated with selective intramucosal colonization by E. coli. E. coli strains associated with GCB have an adherent and invasive phenotype, and are highly similar to strains isolated from CGD and Crohn's disease patients. They Future research initiatives in GC include genomic analysis, E. coli vaccination, gene transfer therapy, and development of antimicrobials against E. coli with better ability to target colonic mucosa and mucosal macrophages.
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10. Baumgart M, Dogan B, Rishniw M, Weitzman G, Bosworth B, Yantiss R, et al. Culture independent analysis of ileal mucosa reveals a selective increase in invasive Escherichia coli of novel phylogeny relative to depletion of Clostridiales in Crohn's disease involving the ileum. ISME J. 2007;1(5):403–418.